This invention is directed is to a coating composition that is used in a cathodic electrocoating process and in particular to a coating composition of an epoxy resin and an acrylic resin that provides smooth finishes having excellent corrosion and chip resistance.
The coating of electrically conductive substrates by an electrodeposition process (also called an electrocoating process) is a well known and important industrial process. Electrodeposition of primers to automotive substrates is widely used in the automotive industry. In this process, a conductive article such as an autobody or an autopart is immersed in bath of a coating composition of an aqueous emulsion of film-forming polymer and acts as an electrode in the electrodeposition process. An electric current is passed between the article and a counter-electrode in electrical contact with the aqueous emulsion, until a desired thickness of coating is deposited on the article. In a cathodic electrocoating process, the article to be coated is the cathode and the counter-electrode is the anode.
Resin compositions used in the bath of a typical cathodic electrocoating process are also well known in the art. These resins are typically made from polyepoxide resins (epoxy resins) which have been chain extended and then an adduct is formed to include amines groups in the resin. Amine groups are usually introduced through reaction of the resin with an amine compound. These resins are blended with a crosslinking agent and then neutralized with an acid to form a water emulsion which is usually referred to as a principal emulsion.
The principal emulsion is combined with a pigment paste, coalescent solvents, water, and other additives to form the electrocoating bath. The electrocoating bath is placed in an insulated tank containing the anode. The article to be coated is the cathode and is passed through the tank containing the electrocoating bath. The thickness of the coating deposited on the article is a function of the bath characteristics, the electrical operating characteristics, the immersion time, and the like.
The coated article is removed from the bath after a certain period of time. The article is rinsed with deionized water and the coating is cured typically in an oven at sufficient temperature to produce a crosslinked coating.
Cathodic electrodepositable resin compositions, coating baths, and cathodic electrodeposition processes are disclosed in Jerabek U.S. Pat. No. 3,922,253 issued Nov. 25, 1975; Wismer et al U.S. No. 4,419,467 issued Dec. 6, 1983; Belanger U.S. No. 4,137,140 issued Jan. 30, 1979 and Wismer et al U.S. No. 4,468,307 issued Aug. 25,1984, the disclosures of which are incorporated by reference.
Important characteristics of finishes formed from electrocoating compositions after baking and curing is the smoothness of the finish and the chip and corrosion resistance of the finish. Also, the stability of the electrocoating bath is important.
The novel electrocoating composition of this invention provides a stable electrocoating bath and finishes that are smooth and chip and corrosion resistant.